1. Field of the Invention
The present invention relates to audio crossover circuits for use with audio speakers, and more particularly to an audio crossover circuit including "fast acting" circuitry for achieving a low-pass crossover slope in excess of 30 dB/octave in less than an octave of the crossover frequency using a minimum number of components.
2. Description of the Prior Art
Audio crossover circuits divide audio signals into different frequency bands or ranges for driving two or more speakers in a speaker system. The crossover circuits apportion the frequency spectrum in such a way that each speaker operates in its optimum frequency range and the entire speaker system reproduces sound with a minimum of distortion.
The frequency at which an audio crossover circuit delivers signals to two speakers operating at adjacent frequency ranges is called the crossover frequency. An audio crossover circuit passes a selected frequency range or band of signals to each speaker and attenuates frequencies that are beyond a speaker's crossover frequency. In this way, each speaker reproduces audio signals only in its optimum frequency range and then "rolls off" near the crossover frequency.
The rate at which a crossover circuit attenuates frequencies delivered to a speaker beyond the crossover frequency is called the crossover slope. Crossover slopes are measured in dB of attenuation per octave and are categorized by their magnitude or "steepness".
Audio crossover circuits typically include high-pass and low-pass filter networks having a plurality of capacitors and inductors. The steepness of an audio crossover circuit's crossover slope is primarily determined by the number of capacitors and inductors used. For example, audio crossover circuits having crossover slopes of 6 dB/octave generally have one inductor or capacitor for each filter network. Audio crossover circuits having crossover slopes of 12 dB/octave generally have two inductors or capacitors for each filter network. In general, each additional component adds approximately 6 dB/octave to the crossover slope.
Steep crossover slopes are desirable for several reasons. For example, crossover circuits with steep crossover slopes attenuate frequencies that are beyond a speaker's effective operating range more rapidly so that the speaker audibly reproduces only audio signals in its optimum frequency range, reducing distortion from signals outside the range. In other words, crossover circuits with steep crossover slopes prevent distortion from too much treble energy being delivered to a low frequency range speaker or woofer and prevent distortion from too much bass energy being delivered to higher frequency range speakers such as mid-range speakers or tweeters.
Another reason steep crossover slopes are desirable is because they allow the operating ranges of the speakers to be extended. Since audio crossover circuits with steep crossover slopes attenuate frequencies that are beyond a speaker's effective operating range rapidly, the "roll off" point where audio signals delivered to the speaker are attenuated by the crossover circuit can be moved closer to the range limit, thus allowing an individual speaker to operate over a wider range of frequencies.
A further reason audio crossover circuits with steep crossover slopes are desirable is because they reduce or eliminate interference between speakers operating at adjacent frequency ranges. Since frequencies that are beyond a speaker's effective operating range are attenuated rapidly by these crossover circuits, the speakers reproduce audio signals in their optimum frequency ranges only without reproducing signals in the frequency ranges of adjacent speakers. This reduces interference between adjacent speakers.
Applicant has discovered that it is also advantageous to produce an audio crossover circuit that is "fast-acting". "Fast acting" crossover circuits reach their maximum crossover slopes faster than prior art crossover circuits. Prior art crossover circuits reach their maximum slope in approximately one octave. Applicant has discovered that a crossover circuit that reaches its maximum crossover slope in one half octave improves speaker performance because frequencies outside of the speaker's optimum operating range are attenuated twice as rapidly. Therefore, all the benefits of a steep crossover slope, as discussed above, are doubled.
A "fast acting", steep crossover slope is especially important on the low-pass side of the crossover because a speaker's natural acoustic output typically does not roll off above the usable frequency range, rather it begins to distort. Conversely, on the high-pass side of the crossover, the natural acoustic output typically rolls off immediately below the usable frequency range, providing the opportunity to naturally augment the crossover slope and speed, and make unnecessary a fast-acting, high slope on the high-pass side. Therefore a cost-effective high-performance crossover design can be achieved by a fast-acting, steep slope on the low-pass side of over 30 dB/octave within one half octave, and using a lower slope, such as 12 dB/octave, on the high-pass side. This asymmetrical circuit design uses the natural roll off below the crossover frequency to augment both the speed and slope of the speaker output, thus resulting in an effective high-pass slope of the speaker output that is symmetrical with the low-pass speaker output. In addition, this design increases the useful range of each speaker on the high-pass side because the crossover point can be moved closer to the natural roll off than in prior art symmetrical circuit designs where high-pass and low-pass slopes are the same.
Prior art attempts to produce audio crossover circuits with steep crossover slopes have been limited by competing interests of cost and performance. To produce economical speaker systems, most audio crossover circuits only utilize a few inductors and capacitors that achieve crossover slopes of 24 dB/octave or less, and because of size limitations, the typical crossover slope is 12 dB/octave or less. Additionally, these prior art audio crossover circuits have not addressed the objective of reaching the maximum crossover slope rapidly, and thus don't reach their maximum slope until more than a full octave. As discussed above, such slow-acting crossover circuits with low crossover slopes result in poor speaker performance since frequencies outside of the speaker's optimum operating range are attenuated too slowly and speakers operating at adjacent frequency ranges interfere with one another.
On the other hand, prior art attempts to produce audio crossover circuits with crossover slopes in excess of 24 dB/octave have been impractical due to high costs and excessive weight. To achieve crossover slopes in excess of 24 dB/octave, the accepted practice is to use a combination of five or more inductors and capacitors per filter. These additional electrical components increase the cost and weight of the crossover circuits and thus limit their utility. Moreover, these prior art audio crossover circuits have not addressed the objective of reaching the maximum crossover slopes rapidly, and thus don't reach their maximum slope until approximately a full octave.
Another limitation of prior art audio crossover circuits is their size. Small speakers are desired to meet tight space requirements of many of today's audio systems. However, speaker manufacturers' attempts to build smaller and lighter speakers have been somewhat limited by the relatively large size of prior art audio crossover circuits. Prior art audio crossover circuits are large because of the number of components and because the inductors are spaced to reduce electrical and magnetic interference therebetween. The spacing of components fails to take advantage of mutual coupling of inductors and results in a larger crossover circuit.
U.S. Pat. No. 5,568,560, which was invented by the present applicant and which is owned by the assignee of the present application, discloses an audio crossover circuit that overcomes many of the limitations described above. Specifically, the audio crossover circuit disclosed in the '560 patent achieves a low pass crossover slope in excess of 24 dB/octave using only four electrical components, reaches its maximum crossover slope in less than an octave of the crossover frequency, and is relatively small and compact.
Although the crossover circuit disclosed in the '560 patent works well with normal audio components, Applicant has discovered that it exhibits undesirable characteristics when it is constructed with high performance audio components having lower resistance values. Higher performance components improve the power utilization of the crossover circuit by reducing the amount of power lost in the crossover; however, they also cause an undesirable "rebound effect" in the crossover slope beyond the first octave above the crossover frequency.
Accordingly, there is a need for an improved audio crossover circuit that can be made with higher performance, low resistance components without suffering from the crossover slope "rebound effect" exhibited by prior art crossover circuits.